The invention relates to carburation devices for internal combustion engines of the type comprising auxiliary throttle means in the intake pipe upstream of driver-actuated main throttle means, the auxiliary throttle means opening automatically and progressively in proportion to the increase in the flow rate of air through the pipe and actuating proportioning means metering the flow rate of fuel drawn into the intake pipe. There exists prior art carburation devices of this kind in which the fuel is sucked into the intake pipe via a duct opening into the pipe at a place where substantially the same degree of vacuum prevails as in the chamber bounded by the two throttle means in the intake pipe.
Usually, fuel is sucked into the intake pipe via a discharge orifice located between the two throttle means. In another device, described in French Patent Specification No. 1,329,682, means are provided which make it possible to deliver the fuel at other places along the intake pipe, more particularly downstream of the main throttle means. In that device, a pipe by-passes the main throttle means, one end of the pipe being connected to the intake pipe between the two throttle means and the other end opening downstream of the main throttle means via a reduced-section orifice, so that the vacuum in the bypass pipe is substantially equal to the pressure between the two throttle means. If the pipe delivering a metered amount of fuel opens near the reduced section, the fuel is introduced into the intake manifold donwstream of the main throttle means but is sucked by a vacuum which is however substantially equal to the vacuum in the intake pipe between the two throttle means.
In such a prior art device, the head loss undergone by the fuel at the pipe mouth must not be excessive as compared with the head loss produced by the fuel metering system (e.g. consisting of a needle having a varying cross-section and movable in a calibrated orifice). Furthermore, since the mouth of the by-pass pipe bringing air downstream of the throttle means must have a larger cross-section than the mouth of the duct supplying fuel, so that the fuel can travel freely through the mouth of the by-pass pipe, the flow cross-section provided by the last-mentioned mouth must be large and consequently an appreciable flow rate of air enters the intake manifold, particularly during idling. The idling conditions thus become hardly adjustable, particularly if such a device is used for each engine cylinder, thus multiplying the number of air inlets by the number of cylinders.
It is an object of the invention to provide an improved carburation device in which the aforementioned disadvantages are overcome, at least to a substantial extent.
According to the invention, there is provided a carburation device of the aforementioned type, wherein the fuel is drawn into the intake pipe downstream of the main throttle means via passage means including a chamber which is kept at a pressure substantially equal to the pressure in the portion of the intake pipe between the two throttle means by a by-pass passage connecting said portion and chamber, having means which are sensitive to at least one engine operating parameter and which, in operation, adjust a control cross-sectional area in the passage means between the chamber and the intake pipe.
The device according to the invention may be regarded as comprising means for modulating the flow control cross-sectional area in accordance with the engine fuel requirements, i.e. for reducing the cross-section at low engine load or speed and increasing it at high engine load or speed. The control cross-sectional area will be the smallest cross-section of the part of the passage means between the chamber and the intake pipe, and is usually placed at the mouth of the passage means into the intake pipe.
The invention will be more clearly understood from the following description of carburation devices given by way of non-limitative examples .